404 BRADFORD ENCRINITES. [Ch. XX. 



after the parasitic serpulce were full grown, they had become incrusted 

 over with a bryozoan, called Diastopora diluviana ; and many gener- 

 ations of these molluscoids had succeeded each other in the pure 

 water before they became fossil. 



We may, therefore, perceive distinctly that, as the pines and cyca- 

 deous plants of the ancient " dirt-bed," or fossil forest, of the Lower 

 Purbeck were killed by submergence under fresh water, and soon 

 buried beneath muddy sediment, so an invasion of argillaceous matter 

 put a sudden stop to the growth of the Bradford Encrinites and led to 

 their preservation in marine strata.* 



Such differences in the fossils as distinguish the calcareous and 

 argillaceous deposits from each other, would be described by natu- 

 ralists as arising out of a difference in the stations of species ; but 

 besides these, there are variations in the fossils of the higher, middle, 

 and lower part of the oolitic series, which must be ascribed to that 

 great law of change in organic life by which distinct assemblages of 

 species have been adapted, at successive geological periods, to the 

 varying conditions of the habitable surface. In a single district it is 

 difficult to decide how far the limitation of species to certain minor 

 formations has been due to the local influence of stations, or how far 

 it has been caused by time or the creative and destroying law above 

 alluded to. But we recognize the reality of the last-mentioned influ- 

 ence, when we contrast the whole oolitic series of England with that 

 of parts of the Jura, Alps, and other distant regions, where there is 

 scarcely any lithological resemblance ; and yet some of the same fossils 

 remain peculiar in each country to the Upper, Middle, and Lower 

 Oolite formations respectively. Mr. Thurmann has shown how remark- 

 ably this fact holds true in the Bernese Jura, although the argillaceous 

 divisions, so conspicuous in England, are feebly represented there, and 

 some entirely wanting. 



The Bradford clay, above alluded to, is sometimes 60 feet thick, but 

 in many places it is wanting ; and in others, where there are no lime- 

 stones, it cannot easily be separated from the clays of the overlying 

 " forest marble " and underlying " fuller's earth." 



The calcareous portion of the Great Oolite consists of several shelly 

 limestones, one of which, called the Bath Oolite, is much celebrated 

 as a building-stone. In parts of Gloucestershire, especially near Min- 

 chinhampton, the Great Oolite, says Mr. Lycett, " must have been de- 

 posited in a shallow sea, where strong currents prevailed, for there are 

 frequent changes in the mineral character of the deposit, and some beds 

 exhibit false stratification. In others, heaps of broken shells are min- 

 gled with pebbles of rocks foreign to the neighborhood, and with frag- 

 ments of abraded madrepores, dicotyledonous wood, and crabs' claws. 

 The shelly strata, also, have occasionally suffered denudation, and the 



* For a fuller account of these Encrinites, see Buckland's Bridgewater Treatise, 

 vol. i. p. 429. 



